This invention relates to an optical amplifier with an improved electronic controller. This controller operates in the gain or output power control mode and works over wide dynamic range of gain or optical signal powers.
In recent years optical amplifiers have undergone considerable transformation. Increased demand for more data transfer resulted in development of wavelength division multiplexing (WDM) technology, which allows more data to be transmitted over one fiber by increased channel count (i.e., a larger number of narrower wavelength ranges within the same predetermined wavelength window). This WDM technology suffers from unwanted effects, such as a variation in gain and output power when the input signal power is constant (for example, due to aging of the amplifier or due to stresses in the amplifier), and cross talk between different channels, for example, when the input signal is modulated at a low frequency. The low frequency is a frequency of up to 100 Hz. This low frequency modulation can be present, for example, due to the addition or dropping of some to the channels, or due to sudden loss of signal at certain wavelengths. These unwanted effects have a negative influence on the power transients (i.e., fluctuations of output optical signal power) of surviving channels, which results in a poor performance of the signal transmission, expressed in an increased bit error rate (BER). In order to minimize the unwanted output signal power fluctuation and the power transients, it is common to introduce a mechanism for controlling either the output signal power or the gain of the optical fiber amplifier. Gain is the ratio of the optical signal output power to the optical signal input power.
There are several known approaches for controlling output signal power or the gain of the optical fiber amplifier. The first approach, known as the electronic feedback/feed-forward approach, utilizes electronic circuitry to control power transients caused by the change of input power, due to adding or dropping of the optical channels. More specifically, amplifier gain or power is controlled by analog tuning of the electronic components, for example by changes resistor""s or capacitor""s values. This approach allows the user, such as a communication company, to minimize power transients in any given optical amplifier by controlling either the amplifier gain or the amplifier output power, but not both. This approach also limits accuracy of gain control when signal power is small. Finally, this approach does not compensate for amplifier noise, such as ASE (amplified spontaneous emission).
The second approach, known as the optical feedback control approach, utilizes only optical components to control power transients of the optical fiber amplifier. This approach is even less flexible than the all-electronic approach described above, because any change in power or gain control requirements requires the change in optical components.
Another approach is to utilize an electronic controller and an additional monitoring channel. This monitoring channel provides a command signal based on information about the change in number of incoming channels. The gain controller, based on these commands, automatically switches between the power control and gain control modes in order to keep amplifier performance within predetermined limits. Furthermore, it is desirable that optical amplifiers operate over a wide dynamic range of input optical power, output optical power or gain while preserving control accuracy. This is especially difficult if a digital controller is utilized and when the input signal is low (less than xe2x88x9228 dBm).
Erbium doped fiber amplifiers (EDFA) are very important components in the optical communication networks. These amplifiers are widely used to provide gain for wavelength division multiplexed optical communications. When high-speed data (over 2 Gbits/sec) is transmitted over the EDFA, the amplifier gain is nearly undisturbed by the fast signal modulation and there is no cross talk between the communication channels. However, low frequency fluctuation of input signal caused, for example, by adding or dropping one or more channels of WDM signal, can cause considerable fluctuation in the inversion levels of Er doped fibers. The negative effects of low frequency fluctuation of the input signal are multiplied if there more than one EDFA is present in the network.
According to one aspect of the present invention an amplifier characterized by gain and output power comprises: (i) at least one gain medium; (ii) at least one pump supplying optical power into the gain medium; (iii) a controller controlling the gain and the output power of the amplifier. The controller includes a signal compression circuit to cover a wide dynamic range for optical input and output signals, so that resolution for low optical signals is better than resolution for high optical signals.
According to one embodiment of the present invention the controller utilizes a logarithmic circuit. According to another embodiment of the present invention the controller utilizes an electronic gain switch circuit.
According to one embodiment of the present invention an amplifier characterized by gain and output power comprises: (i) at least one gain medium; (ii) a pump supplying optical power into said gain medium; (iii) a controller controlling said gain and said output power of said amplifier. This controller includes an electronic gain switch to cover a wide dynamic range for optical input and output signals, so that resolution for low optical signals is better than resolution for high optical signals.
It should be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.